Power trench MOSFETs having SiGe/Si channel structure

ABSTRACT

Devices, methods, and processes that improve immunity to transient voltages and reduce parasitic impedances. Immunity to unclamped inductive switching events is improved. For example, a trench-gated power MOSFET device having a SiGe source is provided, where the SiGe source reduces parasitic npn transistor gain by reducing hole current in the body or well region, thereby decreasing the likelihood of a latch-up condition. A trench-gated power MOSFET device having a SiGe body or well region is also provided. A SiGe body reduces hole current when the body diode is turned on, thereby reducing reverse recovery power losses. Other device characteristics are also improved. For example, parasitic gate impedance can reduced through the use of a poly SiGe gate. Also, channel resistance can be reduced through the use of a SiGe layer near the device&#39;s gate and a thick oxide region can be formed under the trench gate to reduce gate-to-drain capacitance.

This application is a continuation-in-part of U.S. patent application Ser. No. 11/245,995, filed Oct. 7, 2005 and claims the benefit of U.S. provisional application Nos. 60/617,167, 60/617,464, 60/617,463, and 60/617,168, each filed Oct. 7, 2004, all of which are incorporated by reference.

BACKGROUND

The present invention relates generally to power transistors, and more specifically to metal-oxide-semiconductor-gated (MOS-gated) power transistors with silicon-germanium (SiGe) sources, wells, channels, poly silicon-germanium gates, or a combination thereof.

Trench-gated power MOSFET devices are popular choices for use in several demanding applications, such as DC-to-DC converters. These applications can be very harsh, putting a tremendous strain on these transistors. For example, sourcing and sinking large currents into an inductive load can result in large voltage transients at one or more of the device's terminals. Specifically, a large voltage excursion seen by a trench-gated power MOSFET can activate a parasitic npn transistor, leading to a destructive failure. A less catastrophic, but still performance-degrading event, can occur when a large transient forward-biases a device's body diode, slowing the transistor's reverse recovery.

These inductive effects can limit the efficiency of a DC-to-DC converter, thereby wasting power. Other limits on efficiency include the physical limitations of the trench-gated power MOSFETs themselves. For example, parasitic impedances can cause power dissipation and heating of the devices. Among these parasitic impedances is the series resistance of the gates of the devices. While this series resistance can be mitigated using silicide, the efficacy of this procedure is limited due to the physical structure of these transistors. Channel resistance, or Ron, also limits device performance, thus limiting converter efficiency. Larger devices can reduce Ron, but larger devices are more costly.

Accordingly, what is needed are devices, methods, and processes that provide transistors having improved immunity to the effects of large transient voltages and that provide improved performance by reducing parasitic impedances.

SUMMARY

Accordingly, embodiments of the present invention provide devices, methods, and processes that improve immunity to transient voltages and reduce parasitic impedances.

Embodiments of the present invention provide transistors having improved immunity to unclamped inductive switching and other transient voltage events. For example, an exemplary embodiment of the present invention provides a trench-gated power MOSFET device having a SiGe source. A SiGe source reduces the gain of a parasitic npn transistor by reducing hole current in the body or well region, thereby decreasing the likelihood of a latch-up condition following an unclamped inductive switching event. The body tie on this device can also be eliminated in order to reduce transistor cell size.

Another exemplary embodiment of the present invention provides a trench-gated power MOSFET device having a SiGe body or well region. A SiGe body reduces hole current when the body diode is turned on, thereby reducing the power lost during its reverse recovery.

Embodiments of the present invention may also improve device characteristics. For example, an exemplary embodiment of the present invention reduces parasitic gate impedance by incorporating a poly SiGe gate.

Another exemplary embodiment of the present invention provides a channel having reduced channel resistance through the use of a SiGe layer near the device's gate. This layer may be over the drain region, that is, it may extend under the trench gate region. However, extending the SiGe layer under the gate forms a parasitic capacitance from the bottom portion of the trench gate to the SiGe layer.

To reduce this capacitance, and the resulting Miller capacitance during device operation, the SiGe layer under the trench gate can be replaced a thick oxide region. This thick oxide layer may be formed using local oxidation of silicon (LOCOS) or other appropriate techniques. The use of the thick oxide and removal of the SiGe layer under the gate can also reduce the occurrence of shorts between the drain and gate or between the drain and the source. Further, a specific embodiment forms the SiGe layer in the channel using selective epitaxial growth, which can reduce Ge fraction variations.

Improvements that boost immunity to voltage transients and those that enhance device performance are not exclusive. For example, the use of a SiGe body decreases channel impedance, while a SiGe layer in the body region improves body diode reverse recovery. These embodiments may be used to improve n-channel or p-channel devices. Embodiments of the present invention may incorporate one of more of these or the other features described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section of an n-channel trench-gate power MOSFET that is improved by the incorporation of an embodiment of the present invention;

FIG. 2 illustrates a cross section of an n-channel trench-gate power MOSFET having a SiGe source according to an embodiment of the present invention;

FIG. 3 illustrates the energy band structure through the source-to-well region of the device illustrated in FIG. 2;

FIG. 4 is a flowchart of a method of manufacturing a trench-gate power MOSFET having a SiGe source according to an embodiment of the present invention;

FIG. 5 is a cross section of an n-channel trench-gate power MOSFET that is improved by the incorporation of an embodiment of the present invention;

FIG. 6 is a cross section of an n-channel trench-gate power MOSFET having a SiGe well according to an embodiment of the present invention;

FIG. 7 illustrates the net doping and germanium mole fraction of the MOSFET shown in FIG. 6;

FIG. 8 is a flowchart of a method of manufacturing a trench-gate power MOSFET having a SiGe well region according to an embodiment of the present invention;

FIG. 9 is a cross section of an n-channel trench-gate power MOSFET that is improved by the incorporation of an embodiment of the present invention;

FIG. 10 is a cross section of an n-channel trench-gate power MOSFET having a poly SiGe gate according to an embodiment of the present invention;

FIG. 11 illustrates material resistivity as a function of boron concentration for various materials;

FIG. 12 illustrates a DC-DC converter output including a p-channel high-side power-MOSFET device and an n-channel low-side power-MOSFET device;

FIG. 13 illustrates the increase in efficiency provided by the incorporation of an embodiment of the present invention;

FIG. 14 is a flowchart of a method of manufacturing a trench-gate power MOSFET having a poly SiGe gate according to an embodiment of the present invention;

FIG. 15 is a cross section of a p-channel trench-gate power MOSFET having a SiGe channel region according to an embodiment of the present invention;

FIG. 16 shows the hole mobility measured of two different Si_(1-x)Ge_(x) films grown on relaxed Si_(1-x)Ge_(x) virtual substrate;

FIG. 17 illustrates a change in mobility as a function of germanium concentration in a channel region;

FIG. 18 illustrates a valence band offset for a p-channel trench-gate power MOSFET having a SiGe channel region according to an embodiment of the present invention;

FIGS. 19A-19C illustrate a method of manufacturing a p-channel trench-gate power MOSFET having a SiGe channel region according to an embodiment of the present invention;

FIG. 20 is a flowchart of a method of manufacturing a p-channel trench-gate power MOSFET having a SiGe channel region according to an embodiment of the present invention;

FIG. 21 is a cross section of a p-channel trench-gate power MOSFET having a SiGe channel region according to an embodiment of the present invention;

FIG. 22 illustrates the reduction in resistance achieved using an embodiment of the present invention;

FIG. 23 further illustrates the reduction in resistance achieved using an embodiment of the present invention;

FIG. 24 is a table showing reductions in parasitic capacitances for a specific embodiment of the present invention;

FIGS. 25A-25D illustrate a method of manufacturing a p-channel trench-gate power MOSFET having an oxide region under the trench gate and a SiGe channel region according to an embodiment of the present invention; and

FIG. 26 is a flowchart of a method of manufacturing a trench-gate power MOSFET having an oxide region under the trench gate and a SiGe channel region according to an embodiment of the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 is a cross section of an n-channel trench-gate power MOSFET that is improved by the incorporation of an embodiment of the present invention. This device includes n-type source regions 110, a body formed by p-well 120, n-type drain region 130, substrate 160, gates 140, and metal contact 150.

A parasitic npn bipolar transistor is inherent in this structure. Specifically, the parasitic device's emitter is the source region 110, its base is the body or well region 120, while the parasitic device's collector corresponds to the epi region 130. Destructive failure can occur if this parasitic transistor is biased into its forward active mode of operation. This can occur, for example, during avalanche breakdown induced by an unclamped inductive switching (UIS) event. Holes generated by impact ionization can flow through the base formed by the P-Well 120 causing an ohmic voltage drop. If this voltage drop exceeds about 0.6V, the turn-on voltage for the well-to-source junction diode can become forward biased and inject electrons over the potential barrier, thus initiating forward active operation and potential failure.

Specifically, as shown in FIG. 1, an unclamped inductive switching event can pull the source low (1). This causes holes to flow into the body or well 120 (2). The well charges up, or increases in voltage compared to the source 110, turning on the p-well 120 to source 110 junction (3). The result is electron current to flow into the source 110 (4). This causes the parasitic npn to turn on (5), which can lead to device failure.

This propensity for this series of events to occur can be minimized by providing a low resistance path for the removal of holes from the parasitic base and by reducing the current gain of the parasitic npn 160. This may be achieved by contacting the body or p-well with the source connection 150.

But this body contact increases the size of n-channel MOSFET. Thus, there exists a trade-off between device size and immunity to this failure mechanism. A solution to this problem is to employ band-gap engineering techniques to narrow the energy gap of the source region with respect to the well region. This can be achieved by forming the source region using an alloy of Si_(1-x)Ge_(x), where x is the mole fraction of germanium in the alloy, with typical values 0.1<x<0.3. In other embodiments, other concentrations can be used, for example the concentration of germanium can be less than 10 percent, or greater than 30 percent. The SiGe source region can be fabricated by standard techniques such as epitaxial growth or by ion implantation.

FIG. 2 illustrates a cross section of an n-channel trench-gate power MOSFET having a SiGe source according to an embodiment of the present invention. This device includes an n-type source region 210 that is formed from silicon germanium, a body formed by p-well 220, n-type drain region 230, gates 240, metal contact 250, and n substrate 260. As seen in this figure, elimination of the p-well contact allows a significant reduction in cell pitch. This has the additional benefit of reducing on-state resistance and improving current conduction capability by increasing the size of the source region relative to the device of FIG. 1. This figure, as with the other included figures, is shown for illustrative purposes, and does not limit either the possible embodiments of the present invention or the claims. Also, while in this and the other figures an n-channel transistor is shown, embodiments of the present invention can similarly be used to improve p-channel devices. Features shown in other figures may also be included in this figure. For example, the p-well 220 may also be SiGe, while the gate may be poly SiGe.

The use of SiGe in the source 210 allows for holes to flow to the source, reducing the hole current in the body or p-well 220. Specifically, the presence of germanium in the alloy creates a narrowing of the energy gap (Eg) that is roughly proportional to the germanium mole fraction. Most of this band offset occurs in the valence band. This allows holes to easily flow to the source region, providing a path for hole removal reducing the npn gain. Now when an unclamped inductive event pulls the source low (1), the holes flow to the source 210, and the well does not charge (2).

FIG. 3 illustrates the energy band structure through the source-to-well region of the device illustrated in FIG. 2. FIG. 3 was calculated for a germanium mole fraction of 30%. The data show approximately a 0.21 eV reduction of the energy gap of the source with respect to the well, that is, 0.9 eV in the source compared with 1.1 eV in the well or body region. In theory, this leads to an increase in the hole current crossing the junction by a factor ˜exp(Eg/kT). In practice, the improvement is somewhat less than this, since not all the energy band offset occurs in the valence band.

In a specific embodiment of the present invention, the source 210 has a germanium content of 30 percent by mole fraction. A comparison between devices having a silicon source and this SiGe source shows an increase in hole current and reduction in the npn current gain for a device using a SiGe source region. Specifically, at a typical bias of 0.75V, the hole current is increased by about 100× and the parasitic current gain is reduced by a factor of about 500. This data should be considered worst case, since in practice the well region would not be left entirely floating, but would be remotely contacted in the third dimension (perpendicular to the illustrated cross-section of FIG. 2). This reduction in parasitic current gain makes it less likely that the parasitic transistor 160 would conduct current leading to a catastrophic failure during a UIS event.

FIG. 4 is a flowchart of a method of manufacturing a trench-gate power MOSFET having a SiGe source according to an embodiment of the present invention. In this embodiment, a SiGe source is formed in order to achieve a greater immunity to failure caused by an unclamped inductive switching event.

Specifically, in act 410, an epitaxial layer is formed. A SiGe layer is formed in the epitaxial layer in act 420. A trench is cut in the epitaxial layer in act 430, while in act 440, a gate is formed in the trench. The gate is contacted with metal, and the SiGe layer is contacted to form a source region in acts 450 and 460.

These trench-gate power MOSFETs are particularly useful in low-voltage DC-to-DC converter applications due to their low conduction losses and fast switching time. But the source being pulled low is not the only inductive effect these devices are exposed to. During normal operation, the MOSFET source can become positively biased with respect to the drain, leading to turn-on of the parasitic body-to-drain diode (also commonly referred to as the body diode). It should also be noted that while this and the other embodiments of the present invention are well suited to DC-to-DC power converters, other types of systems may incorporate devices that are improved by the incorporation of embodiments of the present invention.

FIG. 5 illustrates this mechanism. This device includes n-type source regions 510, p-type body region 520, n-type drain region 530, gates 540, and metal contact 550. Specifically, the source can be pulled high (1). This causes the body or well diode to turn on (2). When the source returns low (3), holes continue to be injected into the epi 530 or drain. This hole current slows the reverse recovery (4). This leads to energy losses due to both the forward conduction of the diode and also the reverse recovery of the diode. The dissipated power can be roughly approximated as follows:

P_(cond) = V_(f) × I_(out) × t_(deadtime) × F_(sw) $P_{SW} = {{Vin} \times {trr} \times \frac{Irr}{2} \times {Iout} \times {Fsw}}$

Where V_(f) is the body-diode forward voltage, Iout is the diode forward current, Fsw is the switching frequency, trr is the reverse recovery time, and Irr is the reverse recovery current. For a typical n-channel MOSFET, the reverse recovery characteristics are primarily governed by holes injected from the body region (p-type) into the drain region (n-type).

This analysis shows that diode energy losses can be decreased by reducing the diode forward voltage, reverse recovery current, and reverse recovery time simultaneously. Unfortunately, for conventional silicon technology, there exists an inverse relationship between the reverse recovery parameters and the forward voltage. Techniques that improve the forward voltage typically degrade the reverse recovery, while those that improve reverse recovery degrades the forward voltage. This makes their simultaneous reduction impractical using conventional silicon techniques.

Embodiments of the present invention employ band-gap engineering techniques to narrow the energy gap of the body region with respect to the drain epitaxial region. This can be achieved by forming the transistor body region to include an alloy of Si_(1-x)Ge_(x) where x is the mole fraction of germanium in the alloy, with typical values 0.1<x<0.3. In other embodiments, other concentrations can be used, for example the concentration of germanium can be less then 10 percent, or greater than 30 percent. The body region comprising Si_(1-x)Ge_(x) can be fabricated by standard techniques such as epitaxial growth or by ion implantation.

FIG. 6 is a cross section of an n-channel trench-gate power MOSFET having a SiGe well according to an embodiment of the present invention. This device includes n-type source regions 210 and p-type body region 220, which comprises silicon germanium, n-type drain region 230, and gates 240. It should be noted that the p-type body region 220 to metal 250 contact area can be omitted, as it was in FIG. 2 above. Also, the gate may be polysilicon germanium, or poly SiGe.

The presence of germanium in the alloy creates a narrowing of the energy gap (Eg) that is roughly proportional to the germanium mole fraction. The effect of the smaller bandgap in the body region is to increase electron injection at a given forward bias. This in turn implies reduced hole injection for a given current density.

FIG. 7 illustrates the net doping and germanium mole fraction of the MOSFET shown in FIG. 6. The net doping 710 and germanium mole fraction 720 are plotted along the Y-axis as a function of depth along the X-axis. The net doping is typical of that in the trench-gate MOSFET body and drain regions. The germanium profile has a peak mole fraction of 0.15 and width of 100 nm at its peak.

FIG. 8 is a flowchart of a method of manufacturing a trench-gate power MOSFET having a SiGe well region according to an embodiment of the present invention. In this embodiment, a trench-gate power transistor having a SiGe body region is formed.

Specifically, in act 810, an epitaxial layer is formed. A SiGe well is formed in act 820. This layer can be grown on the epitaxial layer formed in act 810, or it may be implanted into the epitaxial layer formed in act 810. A source is formed in the well in act 830.

A trench is formed or cut in the epitaxial layer in act 840, and a gate is formed in the trench in act 850. The gate is contacted, and the source is contacted in acts 860 and 870.

In a specific embodiment of the present invention, the use of silicon germanium results in a significant decrease in hole current as well as a small reduction in the forward voltage. In this embodiment, at a typical bias of 0.75V, the hole current is decreased by a factor of approximately 5 as compared to a silicon well structure. This results in a significant improvement in the reverse recovery characteristics of the transistor. Further, unlike conventional techniques wherein improved reverse recovery is obtained at the expense of increased forward voltage, the improvement in reverse recovery is achieved along with a reduction in the forward voltage.

These trench-gate power MOSFETs can be used in DC-to-DC converter circuits to achieve voltage conversion with a high efficiency, that is, with a minimum loss of power. Some improvements that can be made to these devices to achieve this goal include lowering device on resistance (Rds_(on)), input capacitance (C_(iss)), gate charge (Q_(g) and Q_(gd)), gate impedance (ESR or R_(g)), body diode reverse recovery (T_(rr)), or a combination of the above.

Conventionally, gate impedance reduction is commonly achieved by saturating gate doping or by adding silicide. Unfortunately, due to the large thermal budget (high temperatures) in trench-gate MOSFET devices, large concentrations of dopant in the gate can enhance dopant penetration into the channel (or well or bulk), particularly when gate oxide is thin. This dopant penetration results in a shift in device threshold voltage (V_(th)). As can be seen in the next figure, the effectiveness of adding a silicide layer is also limited.

FIG. 9 is a cross section of an n-channel trench-gate power MOSFET that is improved by the incorporation of an embodiment of the present invention. This cross section includes a device having a gate 940, source 910, body or well region 920, and drain or epi region 930. The gate 940 includes a silicide layer 942. In a trench-gate power MOSFET device, a small gate dimension is desirable for low voltage, low Rds_(on), and low cost. But the narrow gate dimension limits the effectiveness of the silicide layer 942. That is, the portion of the gate 940 below the silicide layer 942 retains the characteristics of polysilicon: it does not benefit from the presence of the silicide layer 942.

Accordingly, embodiments a lower gate resistance to enable faster switching speeds for MOS-gated power transistors without the above drawbacks and limitations. Specifically, embodiments of the present invention provide a lower gate resistance by using a polysilicon germanium (Si_(1-x)Ge_(x)) gate for MOS-gated power transistors such as power trench-gate MOSFET transistors. Incorporating a poly Si_(1-x)Ge_(x) gate offers improved performance as compared to devices utilizing a conventional polysilicon and silicided gate.

For example, the smaller bandgap of Si_(1-x)Ge_(x) results in a larger number of carriers at a given temperature and doping concentration. This in turn reduces gate depletion that shifts the threshold voltage V_(th) and also lowers gate impedance. Moreover, the carrier mobility in poly Si_(1-x)Ge_(x) is over 3 times higher than that of poly silicon. This further lowers gate impedance by 67% and allows the transistor to switch faster.

FIG. 10 is a cross section of an n-channel trench-gate power MOSFET having a poly SiGe gate according to an embodiment of the present invention. This cross section includes a device having a gate 1040, source 1010, body or well region 1020, and drain or epi region 1030. In this embodiment, the entirety of the gate 1040 benefits from the lower resistance of the poly SiGe, as opposed to the narrower improvement provided by the silicide layer 942 in FIG. 9. It should be noted that the p-type body region 1020 to metal 1050 contact area can be omitted, as it was in FIG. 2 above. Also, the source 1010, p-well 1020, or both, may be formed using SiGe.

Poly SiGe gates can also be used to improve electrostatic discharge devices. For example, a Zener device (n⁺p) built using a Si_(1-x)Ge_(x) poly gate is able to withstand higher electrostatic discharge power than that of a conventional poly silicon gate device. High carrier mobility of poly Si_(1-x)Ge_(x) reduced series resistance, particularly in the low doped p-type regions of the Zener, resulting in less ohmic loss and a cooler operation. As with the other embodiments shown, p-channel devices can also be improved. In p-channel devices, boron penetration from a Si_(1-x)Ge_(x) poly gate to body or well via the gate oxide is reduced. This allows the use of thin oxide without threshold voltage instability.

Gate series resistance is greatly reduced using a SiGe poly gate. By way of comparison, a silicided gate only lowers gate resistance by a factor of two in a conventional trenched gate device. This limited decrease is due to the small geometry available for silicide formation over the gate material in a narrow trench. Using poly Si_(1-x)Ge_(x) as gate material reduces the gate impedance by more than 67% under the same gate cross section, thus offering an even better performance without the extra expense and difficulty of using a silicide process. Moreover, poly SiGe remains stable at 1000° C. while the use of a silicided gate limits subsequence process temperature to less than 850° C. due to possible agglomeration. Among other advantages and feature, the use of the poly SiGe gate 1040 eliminates the need for a silicide layer 942 on the gate.

FIG. 11 illustrates material resistivity as a function of boron concentration for various materials. As can be seen, the resistivity of SiGe poly is much less than conventional polysilicon. In this example, the concentration of germanium is 35 percent by mole fraction at boron concentration of 2×10^^19, where ^^ is a symbol for “to the power of.” In other embodiments, the concentration may be higher than 35 percent, or less than 35 percent.

Overall, using poly Si_(1-x)Ge_(x) gate can reduce gate impedance without altering existing layout, and the process is compatible to present silicon technologies. The benefit of a 67% reduction in gate impedance becomes evident when considering the efficiency of a DC-DC converter having a p-channel high-side MOSFET and an n-channel one on the low side as shown in FIG. 12.

FIG. 13 illustrates the increase in efficiency provided by the incorporation of an embodiment of the present invention. The results in FIG. 13 indicate that the peak efficiency increases from 83% to 88%. This equates to a saving of 30% power that would otherwise be lost. Specifically, the peak efficiency for a conventional poly gate 1310 is lower than the peak efficiency for a SiGe gated device 1320. Data in FIG. 13 also suggests that this converter efficiency is approaching to that with n-channel MOSFETs in both the high- and low-side (curve 1330). This significantly reduces the power consumption of gate driver and will enable designers to simplify driver circuits but at the expense of 1-2% efficiency loss at high output current.

Table 1 lists electrical properties for various materials that could be used in forming a MOSFET gate. Again, SiGe poly has a lower resistivity than conventional polysilicon. Using this material as a gate reduces gate series resistance and increases circuit efficiency.

TABLE 1 Electrical properties of various gate materials Gate Material (concentration = Mobility Resistivity 2 × 20¹⁸ cm⁻³) (cm²/V/sec) mΩ-cm p-type crystalline silicon 78 30 p-type poly silicon NA 100 [8]  p-type Si_(0.9)Ge_(0.1) 275 [3] 12 p-type poly silicon (2 × 10¹⁹) NA 7.5 [9] p-type Si_(0.65)Ge_(0.35) (2 × 10¹⁹) NA 2.0 [9]

FIG. 14 is a flowchart of a method of manufacturing a trench-gate power MOSFET having a poly SiGe gate according to an embodiment of the present invention. In this embodiment, a poly SiGe gate is incorporated into a trench-gate power MOSFET device.

Specifically, in act 1410, an epitaxial layer is formed. In act 1420, a well layer is formed in the epitaxial layer. A source is formed in the well in act 1430.

In act 1430, a trench is formed in the epitaxial layer. A poly SiGe gate is formed in the trench in act 1440. The poly SiGe gate and source are contacted in acts 1450 and 1460.

Even with a poly SiGe gate, or a SiGe source, channel impedance limits the performance of trench-gated power MOSFET devices. That is, it is desirable to reduce the on-resistance (Rdson) of MOS-gated power transistors such as n-channel and p-channel power trench-gate MOSFET devices, particularly in low voltage applications. For example, in the application of synchronous DC-DC converter, the gate driver for a p-channel MOSFET consumes less power than a driver for an n-channel device. Thus, it is very desirable to use p-channel devices for the high side driver instead of n-channel devices, as shown in FIG. 12.

But the Rdson for a p-channel device is much higher than a similar n-channel MOSFET, and this limits its application to only small current region. In low voltage applications, channel resistance dominates the device Rdson. The channel resistance (R_(ch)) is:

$R_{ch} = \frac{L}{Z\;\mu_{p}{C_{ox}\left( {V_{G} - V_{T}} \right)}}$ where L is the channel length, Z is the channel width, C_(ox) is the gate oxide capacitance per unit area, V_(G) is the gate voltage and V_(T) is the threshold voltage. To reduce channel resistance, it is desired to shrink channel length, gate oxide thickness, and lower the threshold voltage. Due to the process limitation or sub-threshold issue, these approaches are limited.

In transistors such as the transistor of FIG. 1, the series on-resistance is dominated by the resistivity of the well region 120, particularly the resistivity of the portion of the well region 120 near the gates 140. If this resistivity is reduced, the series on-resistance of the device is reduced. One way to reduce this resistivity is to utilize a material having a higher mobility. Specifically, if the portion of the channel that begins to conduct first, that is the portion near the gates 140, is made of a higher mobility material, the series on-resistance can be reduced. Accordingly, embodiments of the present invention employ strained Si_(1-x)Ge_(x) in its channel area to reduce channel resistance.

FIG. 15 is a cross section of a p-channel trench-gate power MOSFET having a SiGe channel region according to an embodiment of the present invention. This device includes p-type source regions 1510, an n-well or body 1520, p-type drain region 1530, gates 1540, a SiGe layer 1534. The SiGe layer 1534 is isolated from the gates 1540 by a silicon layer 1532. This silicon layer 1532 provides a proper interface to the polysilicon gate. It should be noted that the n-type body region 1520 to metal 1550 contact area can be omitted, as the p-type body region contact was removed in FIG. 2 above. Also, the source 1510 may be formed using SiGe, while the gate 1540 may be poly SiGe.

A layer of Si_(1-x)Ge_(x) layer 1534 is grown commensurately along the sidewall of a silicon trench. Because the lattice constant of Si_(1-x)Ge_(x) is larger than that of silicon, depending on the germanium mole fraction, the Si_(1-x)Ge_(x) layer is under biaxial compressive strain in [100]/[010] directions. A strained Si_(1-x)Ge_(x) planar layer has increased hole mobility compared to that of bulk silicon under either compressive or tensile biaxial strains. This increased mobility results in reduced resistivity in the SiGe layer 15341.

FIG. 16 shows the hole mobility measured of two different Si_(1-x) Ge_(x) films grown on relaxed Si_(1-x)Ge_(x) virtual substrate. The bulk silicon universal mobility curve 1630 is included as a basis of comparison. The hole mobility in Si_(1-x)Ge_(x) channel layer is greatly enhanced due to valence band splitting and shape change of valence band. This enhancement also depends on the germanium concentration relative to that in Si_(1-x)Ge_(x) virtual substrate; indicating a strain related phenomena. The improved hole mobility with 20 at % Ge 1640 and 84 at % Ge 1650 are shown.

FIG. 17 illustrates a change in mobility as a function of germanium concentration in a channel region. For example, a mobility enhancement of a factor of two is observed in Si_(1-x)Ge_(x) film with a germanium concentration of 50 at %. Thus, the device using Si₅₀Ge₅₀ as channel possesses a channel resistance only half of that of the same device using bulk silicon. This mobility enhancement also significantly improves sub-threshold characteristics, which is important for low voltage applications.

FIG. 18 illustrates a valence band offset for a p-channel trench-gate power MOSFET having a SiGe channel region according to an embodiment of the present invention. This offset equals: ΔE _(V)≅0.65x−0.22x ² where x is the germanium content in the film. This offset helps to confine the holes in the strained Si_(1-x)Ge_(x) channel. The parasitic channel resistance from silicon cap layer is then minimized.

FIGS. 19A-19C illustrate a method of manufacturing a p-channel trench-gate power MOSFET having a SiGe channel region according to an embodiment of the present invention. In FIG. 19A, a silicon epitaxial layer 1930 is deposited or grown on a silicon substrate 1960. A trench is then formed. At some point, a well 1920 is implanted, or grown as part of the epi layer 1930.

In FIG. 19B, an epitaxial Si_(1-x)Ge_(x) layer 1934 is deposited, grown, or implanted along the sidewall of trench. The Si_(1-x)Ge_(x) layer's thickness ranges from 1 nm to 100 nm, depending on the germanium concentration. In one embodiment, the germanium concentration ranges from 10 at % through 80 at %. In other embodiments the germanium concentration may be different, for example it may be less than 10 percent by mole fraction, or more than 80 percent by mole fraction. In other embodiments, the range of germanium concentration may be less.

In FIG. 19C, a silicon cap layer 1932 is grown as an epitaxial layer on the top of Si_(1-x)Ge_(x) layer 1934. In one embodiment, the thickness of the silicon cap layer 640 ranges from 1 nm to 10 nm, though in other embodiments it may be thinner or thicker than this range. Alternately, the silicon layer 1932 may remain after SiGe layer 1934 is implanted through it.

FIG. 20 is a flowchart of a method of manufacturing a p-channel trench-gate power MOSFET having a SiGe channel region according to an embodiment of the present invention. In this embodiment, a SiGe layer is grown in the channel region of a trench-gate power MOSFET device.

Specifically, in act 2010 an epitaxial layer is formed. In act 2020, a well layer is formed. A trench is then etched in the epi layer in act 2030. A SiGe layer is grown in act 2040, and in act 2050 a silicon cap is grown. Gate and source regions are formed and contacted in acts 2060 and 2070.

The device shown in FIG. 15 has a SiGe layer 1534 that extends under the trench gate 1540. This forms a capacitance between the bottom of the trench gate 1540 and SiGe layer 1534. This gate-to-drain capacitance increases the Miller capacitance during device operation. Also, the presence of the SiGe layer over the drain increases the likelihood of shorts between the drain and gate or between the drain and source of the device. Moreover, the mole fraction (concentration) of Ge in the SiGe layer can be difficult to control. Accordingly, a specific embodiment of the present invention forms an oxide region under the trench gate. This region may be formed using local oxidation of silicon (LOCOS) or other appropriate techniques. The SiGe layer, as well as a silicon cap layer, can be grown in the trench using selective epitaxial growth. An example of such a device is shown in the following figure.

FIG. 21 is a cross section of a p-channel trench-gate power MOSFET having a SiGe channel region according to an embodiment of the present invention. This device includes p-type source regions 2110, an n-well or body 2120, p-type drain region 2130, gates 2140, a SiGe layer 2134, and an oxide region 2136. The SiGe layer 2134 is isolated from the gates 2140 by a silicon layer 2132. As before, this silicon layer 2132 provides a proper interface to the gate oxide. It should be noted that the n-type body region 2120 to metal 2150 contact area can be omitted, as the p-type body region contact was removed in FIG. 2 above. Also, the source 2110 may be formed using SiGe, while the gate 2140 may be poly SiGe.

The thick oxide layer 2136 is formed in the area below the trench gate 2140. This layer may be formed using LOCOS or other appropriate technique. A layer of Si_(1-x)Ge_(x) layer 2134 is grown commensurately along the sidewall of a silicon trench. This layer, as well as the silicon layer 2132 may be formed by selectively growing an epitaxial layer along the sides of the device trench.

The removal of the SiGe layer not only reduces the gate-to-drain capacitance of the device, as well as the resulting Miller or gain capacitance when the device is operating as a gain device, but provides other benefits as well.

For example, the variation in mole fraction of Ge in the SiGe layer can be difficult to control. Accordingly, various embodiments of the present invention selectively grow a SiGe epitaxial layer along the side of the trench. This provides better control of the Ge mole fraction and reduces its variability. Further, since the SiGe layer does not extend over the drain under the trench gate, the prevalence of shorts that otherwise could occur between the gate and the drain are reduced. Similarly, shorts between the drain and source occur less frequently. As before, the use of SiGe in the channel reduces device resistance. Again, since the lattice constant of Si_(1-x)Ge_(x) is larger than that of silicon, depending on the germanium mole fraction, the Si_(1-x)Ge_(x) layer is under biaxial compressive strain in [100]/[010] directions. A strained Si_(1-x)Ge_(x) planar layer has increased hole mobility compared to that of bulk silicon under either compressive or tensile biaxial strains. This increased mobility results in reduced resistivity in the SiGe layer 2134. A graph is shown in the following figure.

FIG. 22 illustrates the reduction in resistance achieved using an embodiment of the present invention. These curves show a 30 percent reduction in resistance for a device at a gate-to-source voltage of 1.8 Volts.

FIG. 23 further illustrates the reduction in resistance achieved using an embodiment of the present invention. These curves illustrate a similar reduction in resistance as a function gate-to-source bias voltage.

Again, the use of the oxide region 2136 and the removal of the SiGe layer under the trench gate reduces the gate-to-drain stray capacitances. FIG. 24 is a table showing these reductions for a specific embodiment of the present invention.

The features or regions of this device, such as the oxide region 2136 and SiGe layer 2132, can be formed in a variety of ways. One example is shown in the following figures.

FIGS. 25A-25D illustrate a method of manufacturing a p-channel trench-gate power MOSFET having an oxide region under the trench gate and a SiGe channel region according to an embodiment of the present invention. In FIG. 25A, a silicon epitaxial layer 2530 is deposited or grown on a silicon substrate 2560. A trench is then formed. At some point, a well 2520 is implanted, or grown as part of the epi layer 2530.

In FIG. 25B, a region 2536 of silicon dioxide is formed in the areas that will be under the trench gates. Again, this oxide can be grown using LOCOS or other appropriate techniques.

In FIG. 25C, a SiGe layer is grown in the trench regions. In a specific embodiment of the present invention, this layer is a selectively grown epitaxial layer. In other embodiments, other techniques, such as implanting or depositing, may be used. In an exemplary embodiment, the Si_(1-x)Ge_(x) layer's thickness ranges from 1 nm to 100 nm, depending on the germanium concentration, though in other embodiments this thickness may be wider or narrower than this range. In one embodiment, the germanium concentration ranges from 10 at % through 80 at %. In other embodiments the germanium concentration may be different, for example it may be less than 10 percent by mole fraction, or more than 80 percent by mole fraction. In other embodiments, the range of germanium concentration may be less.

In FIG. 25D, a silicon cap layer 2532 is grown as an epitaxial layer on the top of Si_(1-x)Ge_(x) layer 2534. In one embodiment, the thickness of the silicon cap 2532 layer 640 ranges from 1 nm to 20 nm, though in other embodiments it may be thinner or thicker than this range. Alternately, the silicon layer 2532 may be a layer that remains after SiGe layer 2534 is implanted through it.

FIG. 26 is a flowchart of a method of manufacturing a trench-gate power MOSFET having an oxide region under the trench gate and a SiGe channel region according to an embodiment of the present invention. In this embodiment, an oxide region is formed in the region that will be below the trench gates and a SiGe layer is grown in the channel region of a trench-gate power MOSFET device.

Specifically, in act 2610 an epitaxial layer is formed. In act 2620, a well layer is formed. A trench is then etched in the epi layer in act 2630. An oxide region is formed in the trench in act 2640. This may be done using LOCOS or other appropriate method. A SiGe layer is grown in act 2650, and in act 2660 a silicon cap is grown. Gate and source regions are formed and contacted in acts 2670 and 2680.

It will be appreciated by one skilled in the art that this and the other included flowcharts are highly simplified and illustrate only a few fundamental acts in the manufacturing process. Variations of the processes that may be improved by incorporating an embodiment of the present invention are myriad and not required for an understanding of the present invention.

The above description of exemplary embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form described, and many modifications and variations are possible in light of the teaching above. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. For example, it is understood that the doping polarities of the structures shown and described could be reversed and/or the doping concentrations of the various elements could be altered without departing from the invention. As another example, the present invention is shown and described in the context of a vertical MOSFET embodiment, but various embodiments of the present invention can be similarly implemented in other trenched gate structures such as trenched gate IGBTs, lateral trenched-gate MOSFETs, as well as vertical and lateral planar-gate MOSFETs and IGBTs. Also, the various transistor embodiments can be laid out using the well known open-cell or closed-cell configurations. 

1. An MOS-gated transistor comprising: a source region; a body region forming a pn junction with the source region; a drain region forming a pn junction with the body region; a gate extending in a trench; a first layer of conductive material vertically extending between the gate and at least a portion of the body region; and a dielectric layer extending below the gate and between the gate and the drain region, the dielectric layer further extending directly below the vertically-extending first layer of conductive material, wherein the first layer of conductive material has a lower energy gap than the drain region.
 2. The MOS-gated transistor of claim 1 wherein the first layer comprises silicon germanium.
 3. The MOS-gated transistor of claim 1 wherein the first layer comprises Si_(1-x)Ge_(x) where 0.1<x<0.3.
 4. The MOS-gated transistor of claim 1 wherein the first layer comprises approximately between 10 and 30 percent germanium mole fraction.
 5. The MOS-gated transistor of claim 1 further comprising a silicon cap layer vertically extending between the first layer and the gate.
 6. The MOS-gated transistor of claim 1 wherein the dielectric layer is formed using local oxidation of silicon.
 7. The MOS-gated transistor of claim 1 wherein the first layer is formed using selective epitaxial growth.
 8. The MOS-gated transistor of claim 5 wherein the first layer and the silicon cap layer are formed using selective epitaxial growth.
 9. The MOS-gated transistor of claim 1 wherein the gate comprises polysilicon germanium.
 10. An MOS-gated transistor comprising: a gate extending in a trench; a well region of a first conductivity type; a source region of a second conductivity type forming a pn junction with the well region; a silicon germanium layer vertically extending between the gate and at least a portion of the well region; a silicon cap layer vertically extending between the silicon germanium layer and the gate; a drain region of the second conductivity type forming a pn junction with the well region; and a dielectric layer extending below the gate and between the gate and the drain region, the dielectric layer further extending directly below the vertically-extending silicon germanium layer.
 11. The MOS-gated transistor of claim 10 wherein the dielectric layer is formed using local oxidation of silicon.
 12. The MOS-gated transistor of claim 10 wherein the silicon germanium layer and the silicon cap layer are formed using selective epitaxial growth.
 13. The MOS-gated transistor of claim 10 further comprising a gate dielectric layer vertically extending between the gate and the silicon cap layer.
 14. The MOS-gated transistor of claim 10 wherein when the MOS-gated transistor is in an on state, a current flows through the silicon germanium layer between the source and drain regions.
 15. The MOS-gated transistor of claim 1 wherein when the MOS-gated transistor is in an on state, a current flows through the silicon germanium layer between the source and drain regions. 